The correct answer is D) incorrect brush staggering. Incorrect brush staggering can cause uneven wear on the commutator surface of a direct current generator. When the brushes are not properly staggered, they may wear down certain sections of the commutator more than others, leading to an uneven surface. Proper brush staggering ensures even distribution of wear across the commutator, preventing this issue. The other options are incorrect because: A) rapid changes in load can cause commutator issues, but not specifically uneven wear; B) excessive operation at light load may lead to other problems, but not necessarily uneven commutator wear; and C) unequal pole spacing is not a direct cause of uneven commutator wear.